1. Field of the Invention
The present invention relates to a laser apparatus capable of generating vacuum ultraviolet narrow-band laser beams, while maintaining a target wavelength in high precision.
2. Description of a Related Art
Recently, there are drastic changes in integration degrees of semiconductor devices, so that higher improvements are required as to resolution of exposure system used in semiconductor industries. To suppress chromatic aberration occurred in lenses employed in exposure system, semiconductor device exposing operations should be carried out by using narrow spectral light, or beams. To this end, such laser apparatus capable of generating laser beams whose bandwidths are narrowed is expected as light sources of such exposure system. These laser apparatus is required to output laser beams whose bandwidths are narrowed while maintaining constant wavelengths thereof.
Japanese Patent Publication JP-B2-2631569 (relevant to JP-A-3-238329) discloses the following laser apparatus. That is, the wavelength component in the vicinity of the target wavelength of the excimer laser whose bandwidth is narrowed is selected from the reference beam whose spectral distribution is known, and while the selected wavelength component is used as the reference, the wavelength of the excimer laser whose bandwidth is narrowed is kept at the target wavelength during the laser beam output operation.
In future, it is highly expected that integration degrees of semiconductor devices are reached to 4 gigabytes. In this case, as laser beams used in exposure operations, utilization of molecular fluorine laser beams whose bandwidths are made narrow would be highly expected. However, the above-mentioned Japanese Patent Publication JP-B2-2631569 neither discloses, nor teaches such molecular fluorine laser beams whose bandwidth is narrowed.
On the other hand, Japanese Patent Publication JP-A-5-95154 discloses such a laser apparatus that the spectrum of the atomic fluorine laser beam whose bandwidth is not narrowed is employed as the reference, and the molecular fluorine laser beam whose bandwidth is narrowed is output while the wavelength of this molecular fluorine laser beam is maintained at the target wavelength.
However, a molecular fluorine laser beam, or laser light constitutes one sort of vacuum ultraviolet laser beams, the spectrum of which belongs to the vacuum ultraviolet range. On the other hand, an atomic fluorine laser beam constitutes one sort of visible laser beams, the spectrum of which belongs to the visible range. As a result, the target wavelength of the molecular fluorine laser beam whose bandwidth is narrowed is largely deviated from the spectrum of the atomic fluorine laser beam. As a consequence, the laser apparatus disclosed in JP-A-5-95154 can not output the molecular fluorine laser beam whose bandwidth is narrowed, while maintaining the wavelength of the molecular fluorine laser beam at the target wavelength in high precision.
Under such a circumstance, novel laser apparatus is highly expected by which vacuum ultraviolet laser beams whose bandwidths are narrowed, in particular, molecular fluorine laser beams whose bandwidths are narrowed can be output, while maintaining wavelengths of these laser beams at target wavelengths in high precision.
The present invention has been made to solve the above-explained problems, and therefore, has an object to provide such a laser apparatus capable of outputting vacuum ultraviolet laser beams whose bandwidths are narrowed, while maintaining wavelength of these laser beams at target wavelengths in high precision.
To achieve the above-described object, a laser apparatus according to a first aspect of the present invention comprises: a laser oscillator for generating a vacuum ultraviolet laser beam whose bandwidth is narrowed and a reference beam having a spectral distribution of a vacuum ultraviolet band to output both the vacuum ultraviolet laser beam and the reference beam; a wavelength meter for measuring a central wavelength of the vacuum ultraviolet laser beam whose bandwidth is narrowed by using at least one emission light contained in the reference beam as a reference; and control means for controlling the laser oscillator based upon the central wavelength of the vacuum ultraviolet laser beam whose bandwidth is narrowed, which is measured by the wavelength meter, so as to thereby adjust the central wavelength of the vacuum ultraviolet laser beam whose bandwidth is narrowed to a target wavelength.
Also, a laser apparatus according to a second aspect of the present invention comprises: a laser oscillator for generating a vacuum ultraviolet laser beam whose bandwidth is narrowed to thereby output the vacuum ultraviolet laser beam whose bandwidth is narrowed; a reference light source for generating a reference beam having a spectral distribution of a vacuum ultraviolet band to thereby output the generated reference beam; a wavelength meter for measuring a central wavelength of the vacuum ultraviolet laser beam whose bandwidth is narrowed by using at least one emission light contained in the reference beam as a reference; and control means for controlling the laser oscillator based upon the central wavelength of the vacuum ultraviolet laser beam whose bandwidth is narrowed, which is measured by the wavelength meter, so as to thereby adjust the central wavelength of the vacuum ultraviolet laser beam whose bandwidth is narrowed to a target wavelength.
In the laser apparatus according to the present invention, both the vacuum ultraviolet laser beams whose bandwidth is narrowed and the reference beams are generated from the vacuum ultraviolet light. Then, while at least one emission light contained in the reference beam is employed as a reference, a central wavelength of such vacuum ultraviolet laser beams whose bandwidth is narrowed is measured. In general, since line widths of emission light belonging to the vacuum ultraviolet range are on the order of 1 pm and are sufficiently narrow, a central wavelength of vacuum ultraviolet laser beams whose bandwidth is narrowed can be measured in high precision. According to the present invention, even in such a case that the central wavelength of the vacuum ultraviolet laser beams whose bandwidth is narrowed is drifted and are not made stable, such vacuum ultraviolet laser beams whose bandwidth is narrowed can be output while maintaining the central wavelength of these laser beams at a target wavelength in high precision.